The present invention relates to an aircraft, broadly being of the helicopter variety but to be used for crusing at a forward speed that is higher than usually deemed possible. More particularly, the invention relates to a helicopter having a main rotor producing vertical thrust (lift) and a separate propeller drive for providing forward thrust, and further including a yaw moment control and moment compensating control.
Helicopters are often constructed in that the principal rotor provides lift as well as forward thrust. The rotor, moreover, provides also the attitude control of the craft. For a moment compensation and particularly for control of an effective torque about the vertical axis, one usually provides a propeller in the rear portion of the craft whose axis of rotation is transverse to both, the direction of flight and the vertical axis. Helicopters of the type which have just been referred provide optimized hovering properties but as far as forward speed, economy, and range are concerned, they are definitely inferior as compared with normal aerodynamically operated aircraft, i.e. aircraft with aerodynamic lift producing foils (wings). The reason for this limitation in forward speed is to be seen in that for increasing forward speed, the speed of the oncoming flow with reference to that side of the craft in which the rotor blades are moving in forward direction, does approach actually the speed of sound relative to the moving blade itself so that the flow resistance of the blades increases drastically. On the other hand, on the return side of the respective rotor blades, quite often the flow tends to separate from the blade over large areas owing to its rather high angle of attack. Moreover, in the case of very high forward motion, large areas of the rotor blades are actually subject to an oncoming flow from the back.
In order to increase the range of this kind of aircraft, particularly for increasing the cruising speed, a number of different developments have occurred. For example, the so-called advancing blade concept using coaxial rotors in an attempt to provide a relatively fast flying craft. In another development, the rotors can be stopped, and still in another development, or a so-called circulation flow control is provided in that air is blown out through longitudinal slots in the rotor blades. This approach is called the X-wing concept. Also, it has been proposed to provide a rotor blade control for purposes of obtaining higher harmonic blade control with respect to the oscillation frequency provided by the rotation of the helicopter rotor. Still alternatively, rotors with controlled aerodynamic adjusting surfaces have been provided, the adjusting surfaces being arranged along the trailing edge of the rotor blade.
In order to increase the operating range of helicopter, particularly towards higher cruising speeds it is necessary to overcome the forward propagation limitation outlined above which is basically obtained whenever the return running blades experience flow separation to such an extent that they simply can no longer provide the requisite lift.
On the other hand, separate forward thrust producing engines, being separated from the principal rotor, as well as yaw controlling devices for compensating of yaw movements in helicopters, pertain to the general state of the art. In conjunction therewith it is known to provide a pressure producing propeller drive on the fuselage of a helicopter, in front of the control surface. It is also known for helicopters to provide a pivotable tail end propeller to obtain forward thrust or moment compensation, and to provide a rudder being pivotable about a vertical axis, whereby the particular effective rudder surface is arranged upstream for the respective propeller.
Helicopters of the type referred to above also are known to have a compression producing propeller being pivotable about a vertical axis and being arranged in the rear part of the craft for cooperating with the rudder. It is also known to provide tail end equipment for helicopters which include a shrouded propeller for the production of forward thrust and a rudder is disposed in the wake of that latter propeller. Also known is a deflection device for the propeller wake to cooperate with an unshrounded propeller in the tail portion of the fuselage of a helicopter, whereby the deflection device is provided with a pivotable grid of blades. The blades of the grid have a symmetrical profile and are subject to chamber control.